A sensor is generally followed by a conditioner ensuring the conversion of an electrical magnitude at the output of the sensor into an exploitable electrical magnitude, generally a voltage. In the case of physical measurements using a sensor delivering an electric charge under the effect of a signal to be measured, also called “charge source sensor”, such as a piezoelectric sensor, the suitable conditioner is a charge preamplifier.
In reference to FIG. 1, a charge preamplifier includes a phase inverting amplifier AI, herein of the operational type, and a capacitor Cm, called a “storage capacitor”, collecting the charge delivered by the sensor CAP. The sensor CAP is represented by a charge source qc. More precisely, the operational amplifier AI includes an inverting input terminal connected to an output of the sensor CAP, and a non-inverting input terminal connected to a reference voltage. The storage capacitor Cm is placed between the inverting input terminal and the output terminal of the operational amplifier AI. It is noted that the phase inverting amplifier AI could also be of the transconductance type. The measurement of the voltage at the terminals of the storage capacitor Cm is proportional to the charge quantity qc generated by the sensor CAP, and independent of the capacitance of the sensor CAP and the connecting cables.
Discharging, also called “reset”, of the storage capacitor storing the electric charge delivered by the sensor, can be made permanently or in a pulse way, periodic or not, using a reset system placed in a feedback loop of the charge amplifier. It is noted that it is important to avoid the saturation of the operational amplifier and to keep a sufficient dynamic range.
In the case of a permanent reset, it is known to use a reset system including a resistance placed in parallel with the storage capacitor, enabling the storage capacitor to be discharged. The value of the resistance should be selected very high to reduce the noise it supplies. Indeed, the sensor has generally a leakage current Idet depending on parameters such as temperature, electric bias conditions of the sensor, irradiation doses received, etc. This leakage current generates a shot noise of variance 2qIdet, where q is the elementary charge of an electron. If the resistance has a value equal to 2 kT/(q·Idet), that is 52 MOhms for 1 nA, the power of the shot noise is doubled.
But, a resistance of this order of magnitude is not feasible as a pure passive resistance for reasons of a size unsuitable for integrated circuits. In reference to FIG. 2, SRZ active components biased at very low currents are conventionally used, to replace the resistance. The SRZ active component generates a discharging current id controlled by the variations in the output voltage VsPAC of the operational amplifier AI. In the embodiment of FIG. 2, the SRZ active component is a MOS transistor, equivalent to a voltage controlled current generator. The discharging current id is controlled by the deviation between the output voltage VsPAC and a reference value Vref, corresponding to the output voltage at rest of the charge preamplifier. The discharging current id delivered by the SRZ active component supplies the storage capacitor Cm with a charge opposite to the charge generated by the signal and stored in the storage capacitor Cm.
However, the active element adds noises to the measure. Indeed, the noise of an active element directly depends on the current passing through it. In static operation, that is in the absence of a signal, these noises depend on active element bias currents, which are minimized to restrict the noise source. But in dynamic operation, that is after the arrival of a signal, as soon as the electric charge corresponding to the signal has started to be stored in the storage capacitor, the active component is activated and the discharging current it provides at the input of the charge preamplifier increases. This discharging current reaches a maximum value substantially at the same time as the charge stored in the storage capacitor, possibly slightly later if the corresponding electrical path is slower than the charging circuit of the storage capacitor. The current passing through the active element thus changes from a low rest value to a much higher value required for discharging the storage capacitor. This transient current is a noise source the variance of which increases with the amplitude of said current. A low noise reset system can thus be made in the absence of a signal, but it is not possible to hold this property upon measuring a signal.
But, this noise induces a further fluctuation in the signal measurement and this fluctuation is all the more important that the discharging current maximum is close to the time of arrival of the signal at the input of the charge amplifier.